The present invention relates to a vertical deflection circuit for use in a television receiver and, more particularly, to a vertical deflection circuit of a type wherein no vertical output transformer is employed. This type of vertical deflection circuit with no output transformer is generally referred to as an OTL (Output Transformerless) type vertical deflection circuit.
It is known that variation of the pulse width of the flyback pulse produced in a vertical output stage of the vertical deflection circuit is the cause in the raster on the television picture tube, of a white bar, flicker, jitter, line crowding and/or other raster disorders. In addition thereto, in the vertical deflection output circuit where the output stage is composed of a single-ended push-pull amplifier having a vertical output transistor, an excessive load is often imposed on the output transistor and, in an extreme case, the output transistor is destroyed.
One of the major causes for variation of the vertical flyback pulse width is interference by horizontal flyback pulses from the horizontal deflection circuit, in which case sufficient interlaced scanning can not be performed.
In order to avoid these inconveniences, the vertical flyback pulse width must be controlled at a constant width. However, by the reason as will be described with reference to FIG. 1, a prior art vertical deflection circuit in a television receiver fails to satisfy this requirement.
Referring now to FIG. 1 wherein a prior art vertical deflection circuit now under discussion is shown, a circuit arrangement is such that synchronizing pulses emerging from a sync separator 1 are, after having its waveform shaped in an integrator 2, applied to a vertical oscillating circuit 3 which includes a vertical oscillator 4 and a sawtooth deflecting signal generator 5. The vertical oscillator 4 upon receipt of the shaped sync pulses from the integrator 2 is driven to generate a train of pulses synchronized with the vertical synchronizing signal which are subsequently fed to the sawtooth deflecting signal generator 5. The waveform of a sawtooth deflecting signal emerging from the generator 5 is shown in FIG. 2 and this deflecting signal is transferred from the generator 5 to a vertical deflection output circuit 6 which includes a drive circuit 7 for amplifying the sawtooth deflecting signal upon receipt thereof from the generator 5 and a vertical output stage 8 having a vertical deflection coil D.
As shown in FIG. 2, the sawtooth deflecting signal includes rectangular component pulses and the wave-form of an output signal from the vertical output stage 8 which is similar to that of the sawtooth deflecting signal. The portions of the output signal from the vertical output stage 8, which correspond to the rectangular component pulses of the sawtooth deflecting signal are known as flyback pulses.
In the vertical deflection circuit of the above arrangement, the pulse width of the flyback pulses included in the output of the vertical output stage 8 is affected by the pulse width of the rectangular component pulses fed from the oscillator 4 to the output stage 8. More specifically, in the OTL type vertical deflection circuit, since there is no inductance element, such as a vertical output transformer, that determines the pulse width of the flyback pulse, the pulse width of the flyback pulse is mostly determined by the pulse width of the corresponding rectangular component pulse of the sawtooth deflecting signal applied to the output stage 8. Accordingly, variation of the pulse width of the rectangular component pulses produced in the oscillator 4 results in variation of the pulse width of the flyback pulses produced in the output stage 8.
This inconvenience is likely to be avoided if the pulse width of the output pulse from the oscillator 4 is stabilized. However, this stabilization of the pulse width of the output pulse from the oscillator 4 cannot be achieved without difficulties by the following reason.
In the vertical oscillator 4, the start of each output pulse from this oscillator 4 can be accurately determined by the vertical synchronizing signal applied thereto. On the contrary thereto, the time at which the output pulse from the oscillator 4 terminates is solely determined by operational characteristics of the oscillator 4 itself and, therefore, is often adversely affected by and in the presence of external noises. Once the oscillator 4 is adversely affected by and in the presence of the external noises, the duration between the start and termination of the output pulse from said oscillator, that is, the pulse width, varies and, therefore, the problem remains still unsolved.